2,653 research outputs found
Star-disc interactions in a galactic centre and oblateness of the inner stellar cluster
Structure of a quasi-stationary stellar cluster is modelled assuming that it
is embedded in the gravitational field of a super-massive black hole. Gradual
orbital decay of stellar trajectories is caused by the dissipative interaction
with an accretion disc. Gravitational field of the disc is constructed and its
effect on the cluster structure is taken into account as an axially symmetric
perturbation. Attention is focused on a circumnuclear region (r<10^4
gravitational radii) where the effects of the central black hole and the disc
dominate over the influence of an outer galaxy. It is shown how the stellar
system becomes gradually flattened towards the disc plane. For certain
combinations of the model parameters, a toroidal structure is formed by a
fraction of stars. Growing anisotropy of stellar velocities as well as their
segregation occur. The mass function of the inner cluster is modified and it
progressively departs from the asymptotic form assumed in the outer cluster. A
new stationary distribution can be characterized in terms of velocity
dispersion of the stellar sample in the central region of the modified cluster.Comment: Accepted for publication in MNRAS; 12 pages, 10 figure
Frame-dragging effects on magnetic fields near a rotating black hole
We discuss the role of general relativity frame dragging acting on magnetic
field lines near a rotating (Kerr) black hole. Near ergosphere the magnetic
structure becomes strongly influenced and magnetic null points can develop. We
consider aligned magnetic fields as well as fields inclined with respect to the
rotation axis, and the two cases are shown to behave in profoundly different
ways. Further, we construct surfaces of equal values of local electric and
magnetic intensities, which have not yet been discussed in the full generality
of a boosted rotating black hole.Comment: to appear in the proceedings of "The Central Kiloparsec in Galactic
Nuclei (AHAR 2011)", Journal of Physics: Conference Series (JPCS), IOP
Publishin
Of NBOs and kHz QPOs: a low-frequency modulation in resonant oscillations of relativistic accretion disks
The origin of quasi periodic modulations of flux in the kilohertz range (kHz
QPOs), observed in low-mass X-ray binaries, is usually assumed to be physically
distinct from that of the ``normal branch oscillations'' (NBOs) in the
Z-sources. We show that a low-frequency modulation of the kHz QPOs is a natural
consequence of the non-linear relativistic resonance suggested previously to
explain the properties of the high-frequency twin peaks. The theoretical
results discussed here are reminiscent of the 6 Hz variations of frequency and
amplitude of the kHz QPOs reported by Yu, van der Klis and Jonker (2001).Comment: Accepted for publication in PASJ; 4 pages, 1 figur
Interpreting the High Frequency QPO Power Spectra of Accreting Black Holes
In the context of a relativistic hot spot model, we investigate different
physical mechanisms to explain the behavior of quasi-periodic oscillations
(QPOs) from accreting black holes. The locations and amplitudes of the QPO
peaks are determined by the ray-tracing calculations presented in Schnittman &
Bertschinger (2004a): the black hole mass and angular momentum give the
geodesic coordinate frequencies, while the disk inclination and the hot spot
size, shape, and overbrightness give the amplitudes of the different peaks. In
this paper additional features are added to the existing model to explain the
broadening of the QPO peaks as well as the damping of higher frequency
harmonics in the power spectrum. We present a number of analytic results that
closely agree with more detailed numerical calculations. Four primary pieces
are developed: the addition of multiple hot spots with random phases, a finite
width in the distribution of geodesic orbits, Poisson sampling of the detected
photons, and the scattering of photons from the hot spot through a corona of
hot electrons around the black hole. Finally, the complete model is used to fit
the observed power spectra of both type A and type B QPOs seen in XTE
J1550-564, giving confidence limits on each of the model parameters.Comment: 30 pages, 5 figures, submitted to Ap
- …